Slapping problem

I've been slap-popping a lot lately, and I'm having trouble on my Ibanez Ergodyne 605B. The only string that slaps well at all is the B. Whenever I try to slap on E or A, I get this weird harmonic-sounding ringing. The D and G pop perfectly, and all the strings sound fine when I play fingerstyle.

Is there anyone else who's had a similar problem? Any easy fix, or do I need to adjust my technique?

Are you slapping over the harmonic point at the 24th fret position? That might introduce some harmonics. Trying moving where you slap as suggested. Alternatively try a more brushing stroke with your thumb instead of a bouncing movement.